This invention relates to disc drives, and particularly to improving servo bandwidths and margins of disc drives by shifting the mechanical resonance of the actuator system of the disc drive to a frequency that does not interfere with servo bandwidth and/or margins.
Disc drives are employed in computer systems for the storage and retrieval of data. Typically, a disc drive includes one or more rotating discs to which data are written and from which data are read, together with a transducing head that reads data from and/or writes data to concentric tracks on the rotating disc. Typically, the head is mounted to an arm arranged to move substantially radially across the disc to write data to and read data from the disc. An actuator assembly is operable to move the head adjacent the surface of the disc to confront various ones of the concentric tracks on the disc surface.
In a magnetic disc drive, for example, a read/write head assembly is mounted adjacent the end of an actuator arm and is moved substantially radially adjacent the surface of the disc drive of the disc to confront selected concentric tracks on the disc. The write portion of the head assembly includes an inductive head that receives information signals through a coil to generate a magnetic flux that affects orientation of magnetic domains in the recording disc. The read portion of the head is sensitive to changing magnetic fields as the head passes the magnetic domains on the rotating disc. Similarly, an optical disc drive employs an actuator assembly to position an optical head adjacent selected tracks on optical discs, such as CD-ROMs.
There may be any number of disc surfaces to which data are written and from which data are read. In a magnetic disc drive, each disc includes two oppositely disposed disc surfaces, each confronted by a read/write head.
The actuator assembly comprises an E-block pivotally mounted adjacent the rotating discs. A voice coil motor operates the E-block to rotate about its pivot axis. The E-block includes extended actuator arms, with the heads mounted to the distal ends such that the heads move in an arcuate path generally radially across the disc between an outer track diameter and an inner track diameter. The actuator arms are cantilevered from the main body of the E-block and support the heads to aerodynamically xe2x80x9cflyxe2x80x9d a small distance from the surface of the disc.
During a seek operation, the voice coil motor operates the E-block through acceleration and deceleration cycles to move the head between selected tracks. The acceleration and deceleration of the cantilevered arms, and the impulses associated with the changes of acceleration or deceleration, introduce vibration to the arms which is transmitted through the E-block to the remainder of the disc drive structure. The vibration has a resonance frequency based upon the mechanical structure and characteristics of the actuator assembly. For example, the resonance frequency of the actuator assembly of a disc drive might be about 3.5 to 3.7 KiloHertz (KHz). Often the resonance frequency is such as to adversely interfere with the servo bandwidth and/or margins of the disc drive.
Various attempts have been made to minimize or correct for adverse resonance frequencies of the actuator assembly of the disc drive. For example, vibration absorbers, tuned to the resonance frequency of the actuator assembly, are often employed to dampen the resonance frequency, thereby minimizing vibration effect. However, the resonance frequency of an actuator assembly changes as the operating temperature of the disc drive changes. Consequently, mechanical vibration absorbers tuned to a resonance frequency might operate at a given operating temperature (or small range thereof), but not at another. Burnett, in U.S. Pat. No. 4,924,976, proposed an array of such vibration dampers, each tuned to a slightly different frequency, to accommodate the changing resonance frequency of the actuator assembly due to temperature. This approach, however, simply adds to the weight and bulk of the disc drive, sacrificing precious real estate within the disc drive housing that can be used for other, more useful purposes. The present invention addresses these and other problems, and offers other advantages over the prior art.
A disc drive actuator system according to the present invention includes a controller, an actuator plant having a resonance frequency, and a filter. The controller supplies actuator drive signals to the filter which provides a gain of less than unity to the actuator drive signals at the plant resonance frequency and provides a gain greater than unity to the actuator drive signals at a selected frequency different from the plant resonance frequency. The filter thus establishes a system resonance frequency different from the plant resonance frequency by an amount based on the selected frequency.
The actuator plant includes a movable actuator arm supporting a transducer relative to the disc surface, and a motor responsive to drive signals to move the actuator arm to thereby move the transducer relative to the disc surface. The filter comprises an equalizer filter coupled between the controller and the motor for receiving drive signals from the controller and providing filtered drive signals to the motor.
In one form of the invention, the equalizer filter includes a notch filter tuned to the plant resonance frequency to attenuate signals at the plant resonance frequency, and a gain boost at frequencies above the plant resonance frequency to force a system resonance frequency higher than the plant resonance frequency. The gain boost introduces zero or near zero phase loss, resulting in greater gain and phase margins and improved error transfer function responses.